Semiconductor memory device

ABSTRACT

According to one embodiment, a semiconductor memory device includes a memory cell array, a data storage circuit and a control circuit. The data storage circuit holds first data to be written into the memory cell and holds 1 bit data calculated from the first data. The control circuit writes the data of n bits into the memory cell in a first write operation and then executes a second write operation. The control circuit carries out the following control in the second write operation. It reads data stored in the memory cell in the first write operation. It restores the first data based on the data read from the memory cell and the 1 bit data held in the data storage circuit. It writes the restored first data into the memory cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. Ser. No. 14/688,442 filed Apr. 16, 2015,and claims the benefit of priority under 35 U.S.C. § 119 from JapanesePatent Application No. 2014-085018 filed Apr. 16, 2014, the entirecontents of each of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor memorydevice that can store multilevel data.

BACKGROUND

In a semiconductor memory device, e.g., an NAND flash memory, all orhalf of memory cells aligned in a row direction are connected throughbit lines to latch circuits for writing and reading respectively, and awrite or read operation is collectively performed with respect to all orhalf of the memory cells (e.g., memory cells of 2 to 16 kB) aligned inthe row direction.

A write or read unit is called a page, and a block includes pages.Erasing with respect to memory cells is performed in unites of blocks.Electrons are extracted from the memory cells by an erase operation tonegatively set a threshold voltage, and the electrons are introducedinto the memory cells by a write operation, whereby the thresholdvoltage is positively set.

A multilevel memory has been recently developed, which sets one ofthreshold voltages (which are also referred to as threshold levelshereinafter) to one memory cell and stores data of bits. For example,when 4 threshold levels are provided, data of 2 bits can be stored inone cell; and when 8 threshold levels are provided, data of 3 bits canbe stored in one cell. Further, when 16 threshold levels are provided,data of 4 bits can be stored in one cell.

On the other hand, miniaturization for processing a floating gate of anelement has recently become rigorous, a structure that stores electronsin MONOS cells has been also suggested, and density growth realized bythree-dimensional arrangement has been devised in particular. However, athreshold value Vth of the MONOS cells decreases due to detrappingimmediately after a write operation in some cases.

As a countermeasure, there is a method of again performing a verifyoperation after a first write sequence and again carrying out writingwith respect to cells, which have not reached a target threshold value,in accordance with each threshold value. However, this method can solvethe detrapping problem but increases a write time. That is, at the timeof performing rewriting with respect to memory cells that have notreached a target threshold value, one write pulse could be supplied toeach threshold level, but a write threshold value used for writing mustbe held in a data storage circuit in a sense amplifier until this writepulse is supplied, and a cache program for storing subsequent write datain the data storage circuit in advance cannot be executed in some cases.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing a configuration of an NAND flashmemory as a semiconductor memory device according to a first embodiment;

FIG. 2 is a circuit diagram of a memory cell array and a bit linecontrol circuit in the first embodiment;

FIG. 3 is a circuit diagram of another example of the memory cell arrayand the bit line control circuit in the first embodiment;

FIGS. 4A and 4B are cross-sectional views each showing a memory cell anda selection transistor in the first embodiment;

FIG. 5 is a cross-sectional view of an NAND flash memory in the firstembodiment;

FIG. 6 is a cross-sectional view of a memory cell array of the NANDflash memory having a three-dimensional structure;

FIG. 7 is a circuit diagram of the memory cell array of the NAND flashmemory having the three-dimensional structure;

FIG. 8 is a cross-sectional view of another example of the memory cellarray of the NAND flash memory having the three-dimensional structure;

FIG. 9 is a circuit diagram of another example of the memory cell arrayof the NAND flash memory having the three-dimensional structure;

FIG. 10 is a view showing an example of voltages supplied to respectiveregions depicted in FIG. 5;

FIG. 11 is a circuit diagram of a sense amplifier unit in a data storagecircuit in the first embodiment;

FIG. 12 is a circuit diagram of a data control unit in the data storagecircuit in the first embodiment;

FIG. 13 is a view schematically showing a configuration of memoryregions of the memory cell array in the first embodiment;

FIG. 14 is a view showing an example of a writing order of the memorycells in the first embodiment;

FIG. 15 is a view schematically showing threshold voltages written inthe memory cells in a block MLB and read voltage levels in the firstembodiment;

FIG. 16 is a flowchart showing a program sequence in the firstembodiment;

FIG. 17 is a view showing data held in a data latch circuit in theprogram sequence;

FIG. 18 is a view showing an example of a timing chart in the programsequence;

FIG. 19 is a view showing another example of the timing chart in theprogram sequence;

FIG. 20 is an operation waveform chart showing a write operation in theprogram sequence;

FIG. 21 is an operation waveform chart showing a read operation and awrite verify operation in the program sequence;

FIG. 22 is a flowchart showing a program sequence according to amodification of the first embodiment;

FIG. 23 is a view showing data held in data latch circuits in theprogram sequence;

FIG. 24 is a flowchart showing a program sequence according to a secondembodiment;

FIG. 25 is a view showing data held in data latch circuits in theprogram sequence;

FIG. 26 is a flowchart showing a program sequence according to a thirdembodiment;

FIG. 27 is a circuit diagram of a memory cell array of the NAND flashmemory having a three-dimensional structure;

FIG. 28 is a circuit diagram showing another example of the memory cellarray of the NAND flash memory having the three-dimensional structure;

FIG. 29 is a flowchart showing a program sequence according to amodification of the third embodiment;

FIG. 30 is a view showing a write operation and a write verify operationin a program sequence according to a fourth embodiment;

FIG. 31 is an operation waveform chart showing a weak erase operation inthe embodiment; and

FIG. 32 is an operation waveform chart showing another weak eraseoperation in the embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor memory deviceincludes a memory cell array, a data storage circuit and a controlcircuit. The memory cell array includes memory cells each of which canstore data of n bits (n is a natural number equal to 2 or more). Thedata storage circuit holds first data to be written into one of thememory cells and also holds 1 bit data calculated from the first data.The control circuit writes the data of n bits into the memory cell in afirst write operation and then executes a second write operation. Thecontrol circuit carries out the following control in the second writeoperation. It reads data stored in the memory cell in the first writeoperation. It restores the first data based on the data read from thememory cell and the 1 bit data held in the data storage circuit. Itwrites the restored first data into the memory cell.

An embodiment will now be described hereinafter with reference to thedrawings. It is to be noted that like reference numerals denoteconstituent elements having like functions and structures.

First Embodiment

A semiconductor memory device according to a first embodiment will nowbe described.

FIG. 1 shows a configuration of an NAND flash memory as a semiconductormemory device that stores 2 values (1 bit), 4 values (2 bits), or 8values or more (3 bits or more) in each of memory cells.

A memory cell array 1 includes bits lines, word lines, and a commonsource line, and has memory cells, which are formed of, e.g., EEPROMcells and in which data is electrically rewritable, arranged in a matrixform. The memory cell array 1 is connected to a bit line control circuit2 that controls the bit lines and a word line control circuit 6.

The bit line control circuit 2 reads data from the memory cells in thememory cell array 1 through the bit lines. The bit line control circuit2 detects states of the memory cells in the memory cell array 1 throughthe bit lines or applies a write control voltage to the memory cells inthe memory cell array 1 through the bit lines to write data in thememory cells.

The bit line control circuit 2 is connected to a column decoder 3 and adata input/output buffer 4. Data storage circuits in the bit linecontrol circuit 2 are selected by the column decoder 3. Data of thememory cells read to the data storage circuits is output to a controlsection 9 from a data input/output terminal 5 through the datainput/output buffer 4. The data input/output terminal 5 is connected tothe control section 9 outside a memory chip.

The control section 9 includes, e.g., a microcomputer and receives dataoutput from the data input/output terminal 5. Further, the controlsection 9 outputs various kinds of commands CMD that control operationsof the NAND flash memory, an address ADD, and data DT. Write data inputfrom the control section 9 to the data input/output terminal 5 issupplied to a data storage circuit selected by the column decoder 3through the data input/output buffer 4, and a command and an address aresupplied to a control signal and control voltage generation circuit (acontrol circuit) 7.

The word line control circuit 6 is connected to the memory cell array 1.This word line control circuit 6 selects a word line in the memory cellarray 1, and applies a voltage required for reading, writing, or erasingto the selected word line. The memory cell array 1, the bit line controlcircuit 2, the column decoder 3, the data input/output buffer 4, and theword line control circuit 6 are connected to the control signal andcontrol voltage generation circuit 7.

The control signal and control voltage generation circuit 7 controlsoperations of the memory cell array 1, the bit line control circuit 2,the column decoder 3, the data input/output buffer 4, and the word linecontrol circuit 6. The control signal and control voltage generationcircuit 7 is connected to a control signal input terminal 8, andcontrolled by control signals Address Latch Enable (ALE), Command LatchEnable (CLE), Write Enable (WE), and Read Enable (RE) input from thecontrol section 9 through the control signal input terminal 8. Thecontrol signal and control voltage generation circuit 7 generatesvoltages of the word lines or the bit lines at the time of writing dataand also generates a voltage supplied to a well as will be describedlater. The control signal and control voltage generation circuit 7includes a booster circuit like a charge pump circuit, and can generatea program voltage and other high voltages.

The bit line control circuit 2, the column decoder 3, the word linecontrol circuit 6, and the control signal and control voltage generationcircuit 7 constitute a write circuit and a read circuit.

FIG. 2 shows an example of configurations of the memory cell array 1 andthe bit line control circuit 2. NAND units are arranged in the memorycell array 1. One NAND unit includes memory cells MC formed of, e.g., 64EEPROMs connected in series, and selection gates S1 and S2. Theselection gate S2 is connected to a bit line BL0 e, and the selectiongate S2 is connected to a source line SRC. Control gates of the memorycells arranged in each row are connected to word lines WL0 to WL63 incommon. Furthermore, the selection gate S2 is connected to a select lineSGD in common, and the selection gate S1 is connected to a select lineSGS in common.

The bit line control circuit 2 has data storage circuits 10. A pair ofbit lines (BL0 e, BL0 o), (BL1 e, BL1 o) . . . (BLie, BLio), or (BLne,BLno) is connected to each data storage circuit 10.

As indicated by a broken line, the memory cell array 1 includes blocks.Each blocks includes NAND units, and data is erased in, e.g., blocks.Moreover, an erase operation is simultaneously carried out with respectto two bit lines connected to the data storage circuit 10.

Additionally, memory cells arranged every other bit line and connectedto one word line (memory cells in the range surrounded by the brokenline) constitute one sector. Data is written or read in accordance witheach sector. That is, half of the memory cells arranged in a rowdirection are connected to corresponding bit lines. Thus, a write orread operation is executed to each half of the memory cells arranged inthe row direction.

In a read operation, a program verify operation, and a programoperation, one of two bit lines (BLie, BLio) connected to the datastorage circuit 10 is selected in accordance with an address signal(YA0, YA1 . . . YAi . . . , or YAn) supplied from the outside. Further,in accordance with an external address, one word line is selected, andtwo pages indicated by the broken line are selected. The two pages areswitched using an address.

Two pages are selected in case of storing 2 bits in 1 cell, one page isselected in case of storing 1 bit in 1 cell, three pages are selected incase of storing 3 bits in 1 cell, and four pages are selected in case ofstoring 4 bits in 1 cell.

FIG. 3 shows another example of the configuration of the memory cellarray 1 and the bit line control circuit 2 depicted in FIG. 1. In caseof the configuration shown in FIG. 2, two bit lines (BLie, BLio) areconnected to the data storage circuit 10. On the other hand, in theconfiguration shown in FIG. 3, the data storage circuits 10 areconnected to the bit lines respectively, and memory cells arranged inthe row direction are all connected to corresponding bit lines. Thus, awrite or read operation can be executed to all the memory cells arrangedin the row direction.

It is to be noted that both the configuration shown in FIG. 2 and theconfiguration shown in FIG. 3 can be applied in the followingdescription, but an example using FIG. 3 will be described below.

FIGS. 4A and 4B are cross-sectional views each showing a memory cell anda selection transistor. FIG. 4A shows a memory cell. In a substrate 51(a later-described P-type well region 55), n-type diffusion layers 42 asa source and a drain of the memory cell are formed. A floating gate (FG)44 is formed on the P-type well region 55 through a gate insulating film43. A control gate (CG) 46 is formed on the floating gate 44 through aninsulating film 45.

FIG. 4B shows a selection gate. In the P-type well region 55, n-typediffusion layers 47 as a source and a drain are formed. A control gate49 is formed on the P-type well region 55 through a gate insulating film48.

FIG. 5 shows a cross-sectional view of the NAND flash memory. Forexample, in the P-type semiconductor substrate 51, N-type well regions52, 53, and 54 and a P-type well region 56 are formed. In the N-typewell region 52, the P-type well region 55 is formed. A low-voltageN-channel transistor LVNTr constituting the memory cell array 1 isformed in the P-type well region 55. Furthermore, a low-voltageP-channel transistor LVPTr and the low-voltage N-channel transistorLVNTr constituting the data storage circuit 10 are formed in the N-typewell region 53 and the P-type well region 56, respectively.

A high-voltage N-channel transistor HVNTr that connects the bit lines tothe data storage circuits 10 is formed in the substrate 51. Moreover, ahigh-voltage P-channel transistor HVPTr constituting, e.g., a word linedrive circuit is formed in the N-type well region 54. As shown in FIG.5, the high-voltage transistors HVNTr and HVPTr have gate insulatingfilms which are, for example, thicker than the low-voltage transistorsLVNTr and LVPTr, respectively.

FIG. 6 shows a cross-sectional view of a block in a memory cell array ofan NAND type flash memory having a three-dimensional structure called aPipe type. FIG. 6 shows a configuration of one block BLK0 in the memorycell array, but other blocks BLK have the same configuration.

As shown in FIG. 6, the block BLK0 includes string units SU (12 in thisexample). Each string unit SU includes NAND strings 18.

Each NAND string 18 includes, e.g., 8 memory cell transistors MT, theselection transistors ST1 and ST2, and a back gate transistor BT. Aninterlayer insulating film is formed on a conductive layer (which willbe referred to as a back gate layer hereinafter) BG, and semiconductorlayers 20 are formed in the interlayer insulating film. The word linesWL connected to control gates of the memory cell transistors MT areformed around the semiconductor layers 20. The select gate lines SGD andSGS connected to gates of the selection transistors ST1 and ST2 areformed around the semiconductor layers 20 on the word lines WL.Furthermore, one end of each semiconductor layer 20 is connected to thebit line BL, and the other end of each semiconductor layer 20 isconnected to the source line SL.

FIG. 7 is a circuit diagram showing a configuration of the block in thememory cell array in the NAND flash memory having the three-dimensionalstructure. FIG. 7 shows a configuration of one block BLK0 in the memorycell array, but the other blocks BLK have the same configuration.

As described above, the block BLK0 includes string units SU. Moreover,each string unit SU includes (n+1 in this example, n is a natural numberequal to 0 or more) NAND strings 18. Each of the NAND strings 18includes, e.g., 8 memory cell transistors MT (MT0 to MT7), the selectiontransistors ST1 and sT2, the back gate transistor BT, and dummy celltransistors DTD and DTS.

Each memory cell transistor MT includes a stacked gate including acontrol gate and a charge storage layer, and holds data in a nonvolatilemanner. It is to be noted that the number of the memory cell transistorsMT is not restricted to 8, it may be 16, 32, 64, 128, or the like, andthe number is not limited. The back gate transistor BT also includes astacked gate including a control gate and a charge storage layer likethe memory cell transistor MT. However, the back gate transistor BT isnot configured to hold data, but it functions as a simple current pathat the time of writing or erasing data. The memory cell transistors MT,the back gate transistor BT, and the dummy cell transistors DTD and DTSare arranged so that their current paths are connected in series betweenthe selection transistors ST1 and ST2. It is to be noted that the backgate transistor BT is provided between the memory cell transistors MT3and MT4. The current path of the dummy cell transistor DTD on one endside of the series connection is connected to one end of a current pathof the selection transistor ST1, and the current path of the dummy celltransistor DTS on the other end side is connected to one end of thecurrent path of the selection transistor ST2.

Gates of the selection transistors ST1 in the string units SU0 toSU(M−1) are connected to select gate lines SGD0 to SGD(M−1) in common,respectively. Gates of the selection transistors ST2 are connected toselect gate lines SGS0 to SGS(M−1) in common, respectively. On the otherhand, control gates of the memory cell transistors MT0 to MT7 in thesame block BLK0 are connected to word lines WL0 to WL7 in common,respectively. Control gates of the back gate transistors BT areconnected to back gate line BG (BG0 to BG(L−1) in the blocks BLK0 toBLK(L−1), respectively) in common. Control gates of the dummy celltransistors DTD and DTS are connected to word lines WLDD and WLDS incommon, respectively.

That is, the word lines WL0 to WL7 and the back gate line BG areconnected in common among the string units SU in the same block BLK0,whereas the select gate lines SGD and SGS are independent in accordancewith each string unit SU even within the same block BLK0.

Additionally, in the NAND strings 18 arranged in the matrix form in thememory cell array, the other ends of the current paths of the selectiontransistors ST1 in the NAND strings 18 provided in the same column areconnected to any bit line BL in common. That is, the bit line BLconnects the NAND strings 18 in the same block BLK in common, andfurther connects the NAND strings 18 in the blocks BLK in common.Further, the other ends of the current paths of the selectiontransistors ST2 are connected to any source line SL. The source line SLconnects the NAND strings 18 among the strings units SU in common, forexample.

As described above, data of the memory cell transistors MT provided inthe same block BLK are collectively erased. On the other hand, data isread or written collectively with respect to the memory cell transistorsMT connected to any word line WL in common in any string unit SU in anyblock BLK. The unit serves as a “page”.

FIG. 8 is a cross-sectional view of the memory cell array in the NANDflash memory having the three-dimensional structure called an i type.

As shown in the drawing, NAND strings 19 are arranged between the sourceline layer SL and the bit line BL above a semiconductor substrate. Asemiconductor layer 26 in the NAND string 19 has one columnar shape (ani-like shape). The source line layer SL is formed above thesemiconductor substrate, and columnar semiconductor layers 30 a, 30 band 26 are formed on the source line layer. Furthermore, the selectiontransistor ST2, the memory cell transistors MT0 to MT7, and theselection transistor ST1 are formed around the semiconductor layers 30a, 30 b, and 26 from the lower side in the mentioned order. Moreover,the bit line layer BL is formed on the semiconductor layer 30 b.

Gate insulating films 25 a, 25 b, and 25 c are formed on a side surfaceof the semiconductor layer 26, and the word lines WL0 to WL7 are formedon the gate insulating film 25 a. The word lines WL0 to WL7 function asthe control gates of the memory cell transistors MT0 to MT7,respectively. Gate insulating films 29 a and 29 b are formed on sidesurfaces of the semiconductor layers 30 a and 30 b respectively, and theselect gate lines SGS and SGD are formed on the gate insulating films 29a and 29 b, respectively. The select gate lines SGS and SGD function asgates of the selection transistors ST2 and ST1, respectively. It is tobe noted that, in this configuration, the back gate transistor BT is notrequired.

FIG. 9 is a circuit diagram showing a configuration of the block in thememory cell array of the NAND flash memory having the i typethree-dimensional structure. FIG. 9 shows a configuration of one blockBLK0 in the memory cell array, but other blocks BLK have the sameconfiguration.

The block BLK 0 includes (in this example, 2) string units SU0 and SU1.Additionally, each string unit SU includes (in this example, 4) NANDstrings 19. Each of the NAND strings 19 includes, e.g., 64 memory celltransistors MT, the selection transistors ST1 and ST2, and the dummytransistors DTD and DTS. It is to be noted that FIG. 9 shows the dummycell transistors DTD and DTS, but they are omitted in FIG. 8.

Each memory cell transistor MT includes a stacked gate including acontrol gate and a charge storage layer, and holds data in anon-volatile manner. It is to be noted that the number of the memorycell transistors MT is not restricted to 64, it may be 8, 16, 32, 128,or the like, and the number is not limited. The memory cell transistorsMT and the dummy cell transistors DTD and DTS are arranged so that theircurrent paths are connected in series between the selection transistorsST1 and ST2. The current path of the dummy cell transistor DTD on oneend side of the series connection is connected to one end of a currentpath of the selection transistor ST1, and the current path of the dummycell transistor DTS on the other end side is connected to one end of thecurrent path of the selection transistor ST2.

Gates of the selection transistors ST1 in the strings units SU areconnected to the select gate lines SGD0 and SGD1 in common,respectively. Gates of the selection transistors ST2 are connected tothe select gate lines SGS0 and SGS1 in common, respectively. Controlgates of the memory cell transistors MT in the row direction areconnected to the word lines WL0 to WL63 in common, respectively. Controlgates of the dummy cell transistors DTD and DTS are connected to theword lines WLDD and WLDS in common, respectively.

That is, the word lines WL0 to WL63 are connected among the string unitsSU in the same block BLK0 in common, whereas the select gate lines SGDand SGS are independent in accordance with each string unit SU even inthe same block BLK0.

Further, the other ends of current paths of the selection transistorsST1 in the NAND strings 19 arranged in the row direction in the memorycell array are connected to any bit line BL. That is, the bit line BLconnects the NAND strings 19 in the same block BLK in common, andfurther connects the NAND strings 19 in the blocks BLK in common.Furthermore, the other ends of electric paths of the selectiontransistors ST2 are connected to the source line SRC in common.

The configuration of the memory cell array 111 is described in, e.g.,“Three-dimensional Stacked Nonvolatile Semiconductor Memory”, U.S.patent application Ser. No. 12/407,403 filed on Mar. 19, 2009. Moreover,it is also described in “Three-dimensional Stacked NonvolatileSemiconductor Memory”, U.S. patent application Ser. No. 12/406,524 filedon Mar. 18, 2009; “Nonvolatile Semiconductor Memory Device andManufacturing Method Thereof”, U.S. patent application Ser. No.12/679,991 filed on Mar. 25, 2010; and “Semiconductor Memory andManufacturing Method Thereof”, U.S. patent application Ser. No.12/532,030 filed on Mar. 23, 2009. These patent applications areentirely invoked in the specification by reference.

It is to be noted that all of the configuration shown in FIG. 5, theconfiguration shown in FIG. 6, and the configuration shown in FIG. 8 canbe applied to the embodiment.

FIG. 10 shows an example of voltages applied to respective regionsdepicted in FIG. 5. In erasing, program, and read operations, suchvoltages as shown in FIG. 10 are supplied to the respective regions.Here, a voltage Vera is a voltage applied to the substrate in case oferasing data. A voltage Vss is a reference potential, e.g., a groundvoltage, and a voltage Vdd is a power supply voltage. A voltage Vpgmh isa voltage applied to a gate of an N-channel MOS transistor in a rowdecoder at the time of writing data, and also a potential that enablespassing a write voltage Vpgm of the word lines without decreasing by athreshold voltage of the N-channel MOS transistor. That is, the voltageVpgmh is a voltage corresponding to the voltage Vpgm supplied to theword lines+Vth (Vth: the threshold voltage of the N-channel MOStransistor). A voltage Vreadh is a voltage applied to the gate of theN-channel MOS transistor in the row decoder at the time of reading, andalso a potential that enables passing Vread without decreasing by thethreshold voltage of the N-channel MOS transistor. That is, the voltageVreadh is a voltage supplied to the word lines, and also a voltage thatturns to Vread+Vth (Vth: the threshold voltage of the N-channel MOStransistor) at the time of reading.

Besides, there are Vpass as a voltage that is supplied to a word line ofa non-selected cell at the time of writing data and Vread as a voltagesupplied to a non-selected word line at the time of reading data.

Each of FIG. 11 and FIG. 12 shows an example of the data storage circuit10 depicted in FIG. 3. The data storage circuit 10 includes a senseamplifier unit 10 a depicted in FIG. 11 and a data control unit 10 bdepicted in FIG. 12.

As shown in FIG. 11, the sense amplifier unit 10 a includes N-channelMOS transistors (which will be referred to as NMOS hereinafter) 21 to27, P-channel MOS transistors (which will be referred to as PMOShereinafter) 28 and 29, transfer gates 30 and 31, a latch circuit 31,and a capacitor 33. The latch circuit 32 includes, e.g., clockedinverter circuits 32 a and 32 b.

One end of a current path of the NMOS 21 is connected to a node to whichthe power supply Vdd is supplied. The other end of the current path ofthe NMOS 21 is grounded through the transfer gate 30, the NMOS 24, andthe transfer gate 31. One end of a current path of the NMOS 25 isconnected to a connection node between the NMOS 24 and the transfer gate31. The other end of the NMOS 25 is connected to the bit line BLarranged in the memory cell array. Series circuits of the NMOS 22 and 23are connected to the current path of the NMOS 21 in parallel.

Moreover, one end of a current path of the PMOS 28 is connected to anode to which a power supply voltage Vdd is supplied. The other end ofthe current path of the PMOS 28 is connected to an input end of theinverter circuit 32 a constituting the latch circuit 32 through the PMOS29 and grounded via the NMOS 26. An input end of the clocked invertercircuit 32 b cross-connected to the inverter circuit 32 a is connectedto the data control unit 10 b through the NMOS 27. Additionally, a gateof the PMOS 29 is connected to a connection node between the NMOS 22 and23, and one end of the capacitor 33 is connected to this connectionnode. A clock signal CLK is supplied to the other end of this capacitor33.

The control signal and control voltage generation circuit (which will bereferred to as a generation circuit hereinafter) 7 supplies variouskinds of control signals (e.g., signals BLX, LAT, INV, BLC, BLS, HLL,XXL, STB, RST, and NCO) to the inside of the sense amplifier unit 10 aas described below.

The signal BLX is supplied to a gate of the NMOS 21. The signal LAT atan output terminal of the inverter circuit 32 a constituting the latchcircuit 32 is supplied to a gate of the NMOS constituting the transfergate 30. The signal INV at an input terminal of the inverter circuit 32a is supplied to a gate of the PMOS constituting the transfer gate 30.The signal BLC is supplied to a gate of the NMOS 24. Additionally, thesignal BLS is supplied to a gate of the NMOS 25.

The signal INV is supplied to a gate of the NMOS constituting thetransfer gate 31. The signal LAT is supplied to a gate of the PMOSconstituting the transfer gate 31.

The signal HLL is supplied to a gate of the NMOS 22, and the signal XXLis supplied to a gate of the NMOS 23. The signal STB is supplied to agate of the PMOS 28, and the reset signal RST is supplied to a gate ofthe NMOS 26. Further, the signal NCO is supplied to a gate of the NMOS27.

A write operation, a read operation, and a program verify read operationin the sense amplifier unit 10 a will now be briefly described.

(Write Operation)

In case of writing data in the memory cells, the generation circuit 7supplies the following control signals. First, the generation circuit 7sets the signal STB to a high level (which will be referred to as an Hlevel hereinafter), temporarily sets the reset signal RST to the Hlevel, and resets the latch circuit 32. As a result, the signal LAT ofthe latch circuit 32 turns to the H level, and the signal INV of thesame turns to a low level (which will be referred to as an L levelhereinafter).

Then, the generation circuit 7 sets the signal NCO to the H level.Consequently, data is taken into the latch circuit 32 from the datacontrol unit 10 b. If this data is on the L level (“0”), the signal LATis set to the L level, and the signal INV is set to the H level.Furthermore, if the data is on the H level (“1”) indicative ofnon-writing, the data of the latch circuit 32 remains the same, thesignal LAT is held at the H level, and the signal INV is held at the Llevel.

Subsequently, the generation circuit 7 sets the signals BLX, BLC, andBLS to the H level. Then, in case of writing, i.e., if the latch circuithas the L-level signal LAT and the H-level signal INV (writing), thetransfer gate 30 is turned off, the transfer gate 31 is turned on, andthe bit line BL has the ground voltage Vss. In the state, when the wordline has the program voltage Vpgm, data is written into the memory cell.

On the other hand, in case of non-writing, i.e., if the latch circuit 32has the H-level signal LAT and the L-level signal INV, the transfer gate30 is turned on, the transfer gate 31 is turned off, and hence the bitline BL is charged with the power supply voltage Vdd. Here, if the wordline has the program voltage Vpgm, a channel of the memory cell isboosted to a high potential, no data is written into the memory cell.

(Read Operation, Write Verify Operation)

In case of reading data from the memory cell, the generation circuit 7supplies the following control signals. First, the generation circuit 7temporarily sets the reset signal RST to the H level, and resets thelatch circuit 32. Consequently, the signal LAT of the latch circuit 32turns to the H level, and the signal INV of the same turns to the Llevel.

Then, the generation circuit 7 sets the signals BLS, BLC, BLX, HLL, andXXL to predetermined voltages. Consequently, the bit line BL is charged,and Node of the capacitor 33 is charged to the power supply voltage Vdd.Here, if a threshold voltage of the memory cell is higher than a readvoltage level, the memory cell is in an OFF sate, and the bit line BL isheld at the H level. That is, Node is held at the H level. Additionally,if the threshold voltage of the memory cell is lower than the readvoltage level, the memory cell is in an ON state, and an electric chargein the bit line BL is discharged. Thus, the bit line BL changes to the Llevel. Consequently, Node also changes to the L level.

Subsequently, the generation circuit 7 sets the signal STB to the Llevel. Then, if the memory cell is ON, since Node is on the L level, thePMOS 29 is turned on. Consequently, the latch circuit 32 has the H-levelsignal INV and the L-level signal LAT. On the other hand, if the memorycell is in the OFF state, since Node is on the H level, the PMOS 29 isturned off. Consequently, the signal INV of the latch circuit 32 is heldat the L level, and the signal LAT of the same is held at the H level.

Subsequently, the generation circuit 7 sets the signal NCO to the Hlevel. Then, the NMOS 27 is turned on, and data in the latch circuit 32is transferred to the data control unit 10 b.

Further, after the write operation, the program verify operation ofverifying the threshold voltage of the memory cell is carried out. Theprogram verify operation is substantially the same as the readoperation.

FIG. 12 shows an example of the data control unit 10 b in the datastorage circuit 10. The data control unit 10 b shown in FIG. 12 includesan arithmetic circuit 40 and data latch circuits LDL, UDL, and XDL, andan NMOS 41.

The arithmetic circuit 40 includes a bus (which will be referred to asan IBUS hereinafter), transfer gates 42 and 43, a latch circuit 44, anda setting circuit 45. The transfer gates 42 and 43 are connected to bothends of the IBUS and complementarily operate. The latch circuit 44latches data of the IBUS. The setting circuit 45 sets voltage levels ofthe data latch circuits LDL, UDL, and XDL in accordance with data of thelatch circuit 44.

The transfer gate 42 operates based on complementary signals COND andCONS and connects a bus (which will be referred to as SBUS hereinafter)of the sense amplifier unit 10 a with the IBUS. The transfer gate 43operates based on the complementary signals CONS and COND and connectsthe IBUS with a bus (which will be referred to as DBUS hereinafter)having the data latch circuits LDL, UDL, and XDL connected thereto. Thetransfer gate 43 is OFF when the transfer gate 42 is ON, and thetransfer gate 43 is ON when the transfer gate 42 is OFF.

The latch circuit 44 includes PMOS 46 to 49, NMOS 50 to 56, and aninverter circuit 68. A set signal SET is fed to gates of the PMOS 46 andthe NMOS 50. A reset signal REST is supplied to a gate of the PMOS 48. Asignal IFH is supplied to a gate of the NMOS 53. A signal IFL issupplied to a gate of the NMOS 55. A gate of the NMOS 54 is connected tothe IBUS through the inverter circuit 68. Further, a gate of the NMOS 56is connected to the IBUS.

The setting circuit 45 includes PMOS 57 to 60 and NMOS 61 to 64. Asignal FAIL is supplied to a gate of the PMOS 57 and a gate of the NMOS61. The signal FAIL is a signal of a connection node between the PMOS 47and the NMOS 51 as one output terminal of the latch circuit 44. A signalMTCH is supplied to a gate of each of the PMOS 59 and the NMOS 63. Thesignal MTCH is a signal of a connection node between the PMOS 49 and theNMOS 52 as the other output terminal of the latch circuit 44.Additionally, a signal M2HB is supplied to a gate of the PMOS 58, and asignal F2HB is supplied to a gate of the PMOS 60. A signal F2L issupplied to a gate of the NMOS 62, and a signal M2L is supplied to agate of the NMOS 64.

The data latch circuits LDL, UDL, and XDL have the same configuration,and each of these circuits includes a latch circuit 66 and a transfergate 65 that connects the latch circuit 66 to the DBUS. Each transfergate 65 is controlled by a signal BLCA, a complementary signal BLCA_Bthereof, a signal BLCB, a complementary signal BLCB_B thereof, a signalBLCX, and a complementary signal BLCX_B thereof. The data latch circuitXDL is connected to an external IO through the NMOS 41. A signal CSL issupplied to a gate of the NMOS 41.

As described above, the data control unit 10 b holds write data and alsoholds data read from a memory cell at the time of reading.

Write data of 2 bits supplied from the data input/output buffer 4 islatched by, e.g., the data latch circuits LDL and UDL bit by bit throughthe data latch circuit XDL.

The arithmetic circuit 40 shown in FIG. 12 can execute arithmeticoperations such as AND, OR, exclusive NOR, and others with respect todata in the data latch circuits LDL and UDL. For example, in case ofAND, data held in the data latch circuits LDL and UDL is output to theDBUS and the IBUS. In this case, the IBUS is set to the H level onlywhen both pieces of data held in the data latch circuits LDL and UDL are“1”, and the IBUS is set to the L level in any other case. That is, theIBUS is set to “1” at the time of non-writing only, and the IBUS is setto “0” at the time of writing. This data is transferred to the senseamplifier unit 10 a shown in FIG. 11 via the SBUS, thereby performingwriting.

The sense amplifier units 10 a shown in FIG. 11 and the data latchcircuits LDL, UDL, and XDL shown in FIG. 12 may be prepared, and eacharithmetic circuit 40 shown in FIG. 12 may be arranged with respect tothese sense amplifier units 10 a and data latch circuits. Consequently,a circuit area can be reduced.

Operations of the arithmetic circuit 40 can be modified in many ways,various control methods can be applied to, e.g., one logical operation,and the control methods can be changed as required.

The NAND flash memory according to this embodiment is a multilevelmemory. Thus, data of 2 bits can be stored in 1 cell. The 2 bits can bechanged over based on addresses (a first page, a second page). In caseof storing 2 bits in 1 cell, 2 pages are changed over. However, in caseof storing 3 bits in 1 cell, the respective bits are changed over basedon addresses (a first page, a second page, a third page). Further, incase of storing 4 bits in 1 cell, the bits are changed over based onaddresses (a first page, a second page, a third page, a fourth page).

FIG. 13 schematically shows a configuration of memory regions in thememory cell array according to this embodiment. The memory cell array 1has blocks as described above. In this embodiment, each of these blocksMLB includes a multilevel cell (MLC) that stores n bits in one memorycell.

Furthermore, in this embodiment, a description will be given as to acase where writing is collectively performed with respect to all cellsaligned in the row direction as shown in FIG. 3.

FIG. 14 shows an example of an order of writing in memory cells. In theNAND flash memory, memory cells are sequentially selected from thesource line side and subjected to writing. That is, as shown in FIG. 14,the memory cells are selected in the order of “1”, “2”, “3”, and “4”.

FIG. 15 schematically shows threshold voltages and read voltage levelsfor writing in each memory cell in the block MLB. Data in the memorycell turns to a threshold voltage “11” by the erase operation. Data of 2bits, i.e., a lower page as the first page and an upper page as thesecond page is written in the memory cell, whereby the memory cell hasany one of the threshold voltages “10” (“a”), “00” (“b”), and “01”(“c”).

Moreover, read voltage levels “A_R”, “B_R”, and “C_R” between therespective threshold voltages are used for reading data. Verify voltagelevels at the time of writing are A_V, B_V, and C_V which are slightlyhigher than the read voltage levels “A_R, “B_R”, and “C_R” to providedata retention margins, respectively. That is, in this embodiment, theread voltage levels A_R, B_R, and C_R are used in the read operation,and the verify voltage levels A_V, B_V, and C_V are used in the writeverify operation.

A program sequence in the first embodiment will now be describedhereinafter. In the program sequence, as a countermeasure against aproblem that the threshold value of the memory cell decreases due todetrapping immediately after a first write operation, a second writeoperation including a verify operation is performed to restore writedata.

FIG. 16 is a flowchart showing the program sequence in the firstembodiment. FIG. 17 shows data held in the data latch circuit in thisprogram sequence. FIG. 18 shows an example of a timing charge at thetime of writing.

The program sequence in the first embodiment includes the first writeoperation, a weak erase operation, and the second write operation.

(First Write Operation)

As shown in FIG. 18, first, when the H level indicating that a signalReady/Busy is in ready state is set, a data controller in the controlsection 9 loads data of the lower page to the data latch circuit XDL(which will be referred to as XDL hereinafter) shown in FIG. 12. Then,the data in the XDL is transferred to the data latch circuit LDL (whichwill be referred to as LDL hereinafter). Subsequently, the datacontroller in the control section 9 loads data of the upper page to theXDL. Then, the data in the XDL is transferred to the data latch circuitUDL (which will be referred to as UDL hereinafter). In this manner, asrepresented by (a) in FIG. 17, the generation circuit 7 sets the data tothe data latch circuits LDL, UDL, and XDL (a step S1 in FIG. 16).

Subsequently, the first write operation and the write verify operationare carried out based on the program sequence shown in FIG. 16. In thewrite operation, data “a”, “b”, and “c” are written in the memory cells(a step S2). At this time, the generation circuit 7 controls the writeoperation based on the data latched in the LDL. That is, the writeoperation is performed when the data in the LDL is 0, and the writeoperation is not performed when the data in the LDL is 1 (non-writing).

In the verify operation, reading is carried out with respect to thememory cells having the data “a”, “b”, and “c” with the use of theverify voltage levels A_V, B_V, and C_V (steps S3, S4, and S5). As aresult of the verify operation, when a threshold voltage of a memorycell exceeds the verify voltage level, data in the LDL corresponding tothis memory cell is set to “1” from “0”.

Then, whether the memory cells corresponding to data stored in a programbuffer exceed the verify voltage levels is determined. That is, whetherthe number of the data latch circuits LDL set to “1” exceeds a specifiednumber is determined (a step S6). When the number of the data latchcircuits LDL set to “1” exceeds the specified number, the first writeoperation is terminated.

On the other hand, when the number of the data latch circuits LDL set to“1” is less than the specified number, the write voltage Vpgm isslightly increased (step-up), the processing returns to the step S2, andthe write operation of the step S2 and subsequent processing arerepeated.

Consequently, the write operation and the write verify operation arealternately repeated until the number of the data latch circuits LDL setto “1” becomes the specified number or more.

Here, since writing begins from memory cells having lower thresholdvalues in the write operation, when there is no memory cell which is tobe written with the data “a”, the data latch circuit XDL does not haveto latch data for the second write operation. Thus, as shown in FIG. 18,the signal Ready/Busy changes to the H level, and subsequent write datais transferred to the data latch circuit XDL. That is, the data latchcircuit XDL is used as a cache for the subsequent write data.Consequently, high-speed writing can be realized.

After end of the first write operation, the weak erase operation isperformed with respect to the memory cells (a step S10).

The weak erase operation is an operation of applying weak voltage stress(which will be referred to as reverse stress hereinafter) in the samedirection as an erase voltage applied to the memory cells at the time ofa data erase operation. In the weak erase operation, a weak erasevoltage (the reverse stress) in a direction opposite to a write voltageapplied to the memory cells in the first and second write operations isapplied to the memory cells. For example, a voltage of a selected wordline is determined as 0 V, and a channel potential of each memory cellis determined as a positive voltage. Consequently, the reverse stress (areverse pulse) is applied to the memory cell. It is to be noted that adetailed operation of the weak erase operation will be described later.

(Second Write Operation)

As described above, when the specified number or more of the data latchcircuits LDL are set to “1”, the first write operation is completed.Then, the weak erase operation is executed, and the generation circuit 7sets data of 1 bit generated from write data (data of a lower page andan upper page) in the data latch circuits UDL after end of the weakerase operation. For example, the arithmetic circuit 40 executes anarithmetic operation of exclusive NOR with respect to the data in thedata latch circuits LDL and UDL and sets an arithmetic result (“1”, “0”,“1”, and “0”) in the data latch circuits UDL as represented by (a) inFIG. 17.

Then, such a second write operation as shown in FIG. 16 and (b), (c),and (d) in FIG. 17 is executed.

First, to perform writing with respect to the memory cell having thedata “a”, the following operation is executed. As shown in FIG. 16 and(b) in FIG. 17, the memory cell that should have the data “a” is read byusing a read voltage level BC_R (a step S11). The read voltage levelBC_R is a voltage level between the read voltage levels B_R and C_R.When a threshold value of the memory cell subjected to the reading isequal to or less than BC_R and data of the UDL is “0”, data of the LDLcorresponding to the memory cell is set to “0”. As other conditions, thedata of the LDL corresponding to the memory cell is set to “1”.

Then, when the data of the LDL is 0, the generation circuit 7 executesthe verify operation with respect to the memory cell corresponding tothe LDL with the use of the verify voltage level A_V (a step S12).Subsequently, as a result of the verify operation, writing to set thedata “a” is executed with respect to the memory cell whose memory cellthreshold value is equal to the verify voltage level A_V or less (a stepS13).

Subsequently, to again perform writing with respect to the memory cellhaving the data “b”, the following operation is executed. As shown inFIG. 16 and (c) in FIG. 17, the memory cell that should have the data“b” is read by using a read voltage level AB_R (a step S14). The readvoltage level AB_R is a voltage level between the read voltage levelsA_R and B_R. When a threshold value of the memory cell subjected toreading is equal to the read voltage level AB_R or more and data of theUDL is 1, data of the LDL corresponding to the memory cell is set to“0”. As other conditions, the data of the LDL corresponding to thememory cell is set to “1”.

Subsequently, when the data of the LDL is 0, the generation circuit 7executes the verify operation with respect to the memory cellcorresponding to the LDL by using the verify voltage level B_V (a stepS15). Then, writing for setting the data “b” is executed with respect tothe memory cell whose memory cell threshold value is found to be equalto the verify voltage level B_V or less as a result of the verifyoperation (a step S16).

Subsequently, to again perform writing with respect to the memory cellhaving the data “c”, the following operation is executed. As shown inFIG. 16 and (d) in FIG. 17, the memory cell that should have the data“c” is read by using a read voltage level BC_R (a step S17). The readvoltage level BC_R is a voltage level between the read voltage levelsB_R and C_R. When a threshold value of the memory cell subjected toreading is equal to the read voltage level BC_R or more and data of theUDL is 0, data of the LDL corresponding to the memory cell is set to“0”. As other conditions, the data of the LDL corresponding to thememory cell is set to “1”.

Here, the data latch circuit UDL does not have to latch data for thesecond write operation. Thus, if subsequent write data (corresponding to1 page) has been transferred to the data latch circuit XDL from theoutside, the data in the data latch circuit XDL is transferred to thedata latch circuit UDL. Therefore, as shown in FIG. 18, the signalReady/Busy changes to the H level, and the subsequent write data(corresponding to 1 page) is transferred to the data latch circuit XDL.That is, the data latch circuits XDL and UDL are used as caches for thesubsequent write data corresponding to 2 pages. Consequently, high-speedwriting can be realized.

Subsequently, when data in the LDL is 0, the verify operation isexecuted with respect to a memory cell corresponding to the LDL by usingthe verify voltage level C_V (a step S18). Then, in the verifyoperation, writing to set the data “c” is executed with respect to thememory cell whose memory cell threshold value is equal to the verifyvoltage level C_V or less (a step S19).

According to the first embodiment described above, to restore the writedata written in the memory cells having the data “a”, “b”, and “c”,which one of the data “a”, “b”, and “c” each of the read memory cellshas is determined based on a result of reading using the voltage AB_Rbetween the read voltage levels A_R (the read voltage level for the data“a”) and B_R (the read voltage level for the data “b”) and the voltageBC_R between the read voltage levels B_R and C_R (the read voltage levelfor the data “c”), and writing is executed with respect to each of thevoltage levels of the data “a”, “b”, and “c”. As a result, it ispossible to provide the semiconductor memory device that can suppress aninfluence of fluctuations in threshold values immediately after writingin the memory cells and enables high-speed writing.

It is to be noted that, in the first embodiment, although the writeoperation is executed after inputting data corresponding to 2 pages,i.e., data 0L of the lower page and data 0U of the upper page, since thedata “a” can be written when the data of the lower page is input asshown in FIG. 15, writing of the data “a” may be first started, then thedata of the upper page may be input, and thereafter the data “a”, “b”,and “c” may be written. FIG. 19 shows a sequence chart of this example.As shown in the drawing, before inputting the data 0L of the lower pageand inputting the data 0U of the upper page, the write operation may bestarted.

Furthermore, the verify voltage level in the second write operation maybe replaced with the verify voltage level effected in the first writeoperation. Moreover, the program voltage Vpgm and a pulse time forapplying the program voltage Vpgm may be changed. Moreover, after thesecond write operation, the second write operation may be repeated morethan once.

Additionally, if the memory cell has a threshold value that is unstabledue to, e.g., detrapping of an electric charge by performing the weakerase operation after the first write operation and before the secondwrite operation, the threshold value of the memory cell is lowered atthis point of time. Further, rewriting is executed with respect to thememory cell in the second write operation. Consequently, the writeoperation in which a threshold value distribution of the memory cellhardly decreases can be carried out.

In the program sequence shown in FIG. 16, although the weak eraseoperation of the step S10 is executed between the step S6 and the stepS11, the weak erase operation may be executed between the step S11 andthe step S12. The weak erase operation may be executed with respect tothe memory cell which passed the verify operation in the first writeoperation. The weak erase operation also may be executed with respect tothe memory cell in which data “a”, “b” or “c” to be written.

Operation waveforms of the write operation, the read operation, and thewrite verify operation in the program sequence according to theembodiment will now be described hereinafter.

FIG. 20 and FIG. 21 show operation waveforms of the write operation, theread operation, and the write verify operation in the program sequence.

For example, in the write operation of the step S2 shown in FIG. 16, thewrite voltage Vpgm is applied to a selected word line as shown in FIG.20. In the write verify operation of the steps S3, S4, and S5, theverify voltage levels A_V, B_V, and C_V are applied to selected wordlines, respectively as shown in FIG. 21.

Furthermore, in the write verify operation of the steps S11 and S17shown in FIG. 16, the read voltage level BC_R between the read voltagelevels B_R and C_R is applied to a selected word line as shown in FIG.21. In the write verify operation of the step S14, the read voltagelevel AB_R between the read voltage levels A_R and B_R is applied to aselected word line as shown in FIG. 21.

In the write verify operation of the steps S12, S15, and S18, the sameverify voltage levels A_V, B_V, and C_V as those of the steps S3, S4,and S5 may be applied, or slightly different values may be applied toselected word lines as shown in FIG. 21.

A modification of the first embodiment will now be described.

FIG. 22 is a flowchart showing a program sequence according to themodification of the first embodiment. FIG. 23 shows data held in thedata latch circuits in the program sequence. Other structures are thesame as those in the first embodiment.

A first write operation and a weak erase operation in the programsequence according to this modification are the same as the firstembodiment, and hence a description thereof will be omitted. Here, asecond write operation executed after the weak erase operation will bedescribed. It is to be noted that particulars of the weak eraseoperation will be described later.

(Second Write Operation)

As described above, when a specified number or more of the data latchcircuits LDL are set to “1”, a first write operation is completed. Then,the weak erase operation is executed, and “1”, “0”, “1”, and “0” are setto the data latch circuit UDL after end of this weak erase operation asrepresented by (a) in FIG. 23 like the first embodiment. Thereafter,such a second write operation as shown in FIG. 22 and (b), (c), and (d)in FIG. 23 is executed.

First, to again perform writing in each memory cell having data “a”, thefollowing operation is executed. In this modification, as shown in FIG.22 and (b) in FIG. 23, after end of the first write operation, theverify operation is executed using the verify voltage level A_V (a stepS12).

In the first embodiment, as represented by (b) in FIG. 17, after end ofthe first write operation, the memory cells are read by using the datastored in the data latch circuits UDL and the read voltage level BC_R,thereby classifying the memory cells having the data “a” and “c”.Additionally, the verify operation is executed with the use of theverify voltage level A_V to select the memory cell having the data “a”whose threshold value is lower than the verify voltage level A_V.However, it is unlikely that the threshold value of the memory cellhaving the data “c” decreases to reach the verify voltage level A_V orless and, even if writing to set the data “a” is performed with respectto the memory cell having the data “c” equal to the verify voltage levelA_V or less, an influence on the memory cell having the data “c” issmall. Thus, in this example, reading using the read voltage level BC_Ris eliminated, and the verify operation is executed with the use of theverify voltage level A_V.

As shown in FIG. 11, the data latch circuits SDL (32 a, 32 b) (whichwill be referred to as SDL hereinafter) are prepared in the data storagecircuit 10. In the data latch circuits SDL, “0” is latched when athreshold level of each memory cell is lower than the verify voltagelevel A_V as a result of the verify operation using the verify voltagelevel A_V, or “1” is latched when the threshold level of each memorycell is higher than the verify voltage level A_V as a result of thesame.

Subsequently, the data latched in the UDL and the SDL are subjected toan OR operation, and its result is stored in the SDL.

Then, when data in the SDL is 0, namely, the generation circuit 7executes writing to set the data “a” with respect to memory cells whichare to be written with the data “a” or the data “c” have memory cellthreshold values equal to the verify voltage level A_V or less (the stepS13). Subsequent operations represented by (c) and (d) in FIG. 23 arethe same as the first embodiment.

In the modification according to the first embodiment, omitting the readoperation for discriminating the memory cells which are to be writtenwith the data “a” and the memory cells which are to be written with thedata “c” enables reducing the operations in the program sequence beyondthe first embodiment, thereby realizing high-speed writing. Otherstructures and effects are the same as the first embodiment.

Second Embodiment

A semiconductor memory device according to a second embodiment will nowbe described.

FIG. 24 is a flowchart showing a program sequence according to thesecond embodiment. FIG. 25 shows data held in data latch circuits at thetime of the program sequence. Other structures are the same as the firstembodiment.

A first write operation and a weak erase operation in the programsequence according to the second embodiment are the same as the firstembodiment, and a second write operation executed after the weak eraseoperation will be described here. It is to be noted that particulars ofthe weak erase operation will be described later.

(Second Write Operation)

As described above, when a specified number or more of data latchcircuits LDL are set to “1”, the first write operation is completed.Then, the weak erase operation is executed, and “1”, “0”, “1”, and “0”are set to a data latch circuit UDL after end of the weak eraseoperation as represented by (a) in FIG. 25 like the first embodiment.Thereafter, such a second write operation as shown in FIG. 24 and (b),(c), and (d) in FIG. 25 is executed.

Here, the data latch circuit LDL does not have to latch data for thesecond write operation. Thus, if next write data (corresponding to 1page) has been transferred to a data latch circuit XDL from the outside,data in the data latch circuit XDL is transferred to the data latchcircuit LDL. Therefore, as shown in FIG. 18, a signal Ready/Busy is setto an H level, and the next write data (corresponding to 1 page) istransferred to the data latch circuit XDL. That is, the data latchcircuits XDL and LDL are used as caches for the next write datacorresponding to 2 pages. Consequently, high-speed writing can berealized.

First, to again perform writing with respect to a memory cell having thedata “a”, the following operation is executed. In this secondembodiment, as shown in FIG. 24 and (b) in FIG. 25, after completion ofthe first write operation, like the modification of the firstembodiment, a verify voltage level A_V is used to execute a verifyoperation (a step S12).

In the data latch circuits SDL, “0” is latched when a threshold level ofthe memory cell is lower than the verify voltage level A_V as a resultof the verify operation using the verify voltage level A_V, or “1” islatched when the threshold level of the memory cell is higher than theverify voltage level A_V as a result of the same.

Then, a result of performing an OR operation to data latched in the UDLand the SDL and a result of performing the OR operation to inverted dataof the data latched in the UDL and the SDL are stored in the SDL and theUDL, respectively. Here, the UDL turns to 0 when the UDL originallyholds data “1” (i.e., a memory cell of write Inhibit and a cell which isto be written with data “b”) and has the verify voltage level A_V or ahigher level, but the memory cell of write Inhibit is unlikely to have athreshold value of the verify voltage level A_V or a higher level, onlythe cell which is to be written with the data “b” remains.

Then, a generation circuit 7 executes writing to provide the data “a”when the data in the SDL is 0, namely, with respect to a memory cellwhich is to be written with the data “a” or the data “c” and has amemory cell threshold value equal to the verify voltage level A_V orless (a step S13).

Then, to again perform writing with respect to the memory cell havingthe data “b”, the following operation is executed. As shown in FIG. 24and (c) in FIG. 25, the verify operation is executed with the use of averify voltage level B_V (a step S15). In the data latch circuit SDL,“0” is latched when a threshold level of the memory cell is lower thanthe verify voltage level B_V as a result of the verify operation usingthe verify voltage level B_V, or “1” is latched when the threshold levelof the memory cell is higher than the verify voltage level B_V as aresult of the same.

Subsequently, a result of performing the OR operation to the datalatched in the UDL and the SDL and a result of performing the ORoperation to inverted data of the data latched in the UDL and SDL arestored in the SDL and the UDL, respectively. Here, the UDL turns to 0when the UDL originally holds data “1” (i.e., a memory cell of writeInhibit or cells which are to be written with data “a” and “c”) and hasthe verify voltage level B_V or a higher level, but the memory cell ofwrite Inhibit or the cell which is to be written with the data “a” isunlikely to have a threshold value of the verify voltage level B_V or ahigher level, only the cell which is to be written with the data “c”remains.

Then, the generation circuit 7 executes writing to provide the data “b”when the data in the SDL is 0, namely, with respect to the memory cellthat is to be written with the data “b” and has a memory cell thresholdvalue equal to the verify voltage level B_V or less (a step S16).

Then, to again perform writing with respect to the memory cell havingthe data “c”, the following operation is executed. As shown in FIG. 24and (d) in FIG. 25, the verify operation is executed with the use of averify voltage level C_V (a step S18). In the data latch circuit SDL,“0” is latched when a threshold level of the memory cell is lower thanthe verify voltage level C_V as a result of the verify operation usingthe verify voltage level C_V, or “1” is latched when the threshold levelof the memory cell is higher than the verify voltage level C_V as aresult of the same.

Subsequently, the data latched in the UDL and the SDL are subjected tothe OR operation, and its result is stored in the SDL and the UDL.

Then, the generation circuit 7 executes writing to provide the data “c”when the data in the SDL is 0, namely, with respect to a memory cellwhich is to be written with the data “c” and has a memory cell thresholdvalue equal to the verify voltage level C_V or less (a step S19).

Here, the data latch circuit UDL does not have to latch data for thesecond write operation. Thus, the UDL can be used as, e.g., a data latchcircuit that holds next write data.

Further, the verify voltage level of the second write operation may bereplaced with the verify voltage level effected in the first writeoperation. Moreover, a program voltage Vpgm and a pulse time forapplying the program voltage Vpgm may be changed.

After the writing to provide the data “c”, the program sequence isterminated, but the second write operation may be repeated more thanonce.

In the second embodiment, the reading using the read voltage level AB_Rin the first embodiment uses a result of the verify operation adoptingthe verify voltage level A_V and the reading using the read voltagelevel BC_R uses a result of the verify operation adopting the verifyvoltage level B_V in the same, thereby omitting a time required forpreliminary reading adopting the read voltage levels AB_R and BC_R.Consequently, high-speed reading can be realized. Other structures andeffects are the same as those in the first embodiment.

Third Embodiment

A semiconductor memory device according to a third embodiment will nowbe described. In the third embodiment, the first write operation and thesecond write operation described in the first embodiment are executed indifferent sequences.

(a) and (b) in FIG. 26 are flowcharts showing program sequencesaccording to the third embodiment. A first program sequence representedby (a) in FIG. 26 is the same as the first write operations in the firstembodiment and the modification of the first embodiment or the secondembodiment. A second program sequence represented by (b) in FIG. 26 isthe same as the second write operation in the first embodiment. It is tobe noted that, in this example, the second write operation in themodification of the first embodiment or the second embodiment may beexecuted in place of the second program sequence represented by (b) inFIG. 26.

Based on the first program sequence represented by (a) in FIG. 26, dataof 2 bits, i.e., data “a”, “b”, and “c” are written in memory cells, andthe first program sequence is terminated. Then, after a fixed period oftime, data of 2 bits are again written in the same memory cells based onthe second program sequence represented by (b) in FIG. 26.

In the second program sequence, the second write operation is executedby using data of 1 bit generated from the write data of 2 bits. Althoughthe write data in the second program sequence may be held in the datastorage circuits shown in FIG. 2 or 3, a latch circuit may be newlyprovided to hold the data. If the latch circuit is newly provided and achip size disadvantageously increases, the write data in the secondprogram sequence may be stored in some of the memory cells as 2-valuedata. Furthermore, the data may be again transferred to the memoriesfrom an external controller. In any case, data corresponding to 2 bitsis not required, and only data of 1 bit generated from 2 bits cansuffice, thereby suppressing a data transfer time and a memory region.

The second program sequence may be executed after writing data in amemory cell adjacent to a writing target memory cell. An example of sucha case will now be described hereinafter with reference to FIG. 27.

FIG. 27 is a circuit diagram showing a configuration of a memory cellarray of an NAND flash memory having a three-dimensional structurecalled an i type. NAND strings 19 are arranged in the memory cell array.One NAND string 19 includes, e.g., memory cells MC formed of, e.g., 64series-connected EEPROMs, dummy cell transistors DTD and DTS, andselection transistors SGS and SGD. The selection transistors SGS0 toSGS3 are connected to ends of the series-connected memory cells MC,respectively. The selection transistors SGD0 to SGD3 are connected tothe other ends of the memory cells MC, respectively.

Ends of the NAND strings 19 are connected to a source line SRC. Theother ends of the NAND strings 19 are connected to a bit line BL.Moreover, control gates of the memory cells MC arranged in each rowdirection are connected to word lines WLDS, WL1 to WL63, and WLDD incommon, respectively.

It is to be noted that gates of the selection transistors SGS0 to SGS3may be connected to a common wiring SGS and the memory cell strings maybe selected by the selection transistors SGD0 to SGD3, respectively.Additionally, both the selection transistors SGS and SGD may be formedof memory cell transistors having the same MONOS structure as those ofthe memory cells, respectively.

In the memory cell array having such a configuration, based on the firstprogram sequence, writing is executed in the memory cells in the orderof (1), (2), (3), and (4) as shown in FIG. 27. Then, based on the secondprogram sequence, writing is again executed in the memory cells in theorder of (1), (2), (3), and (4).

Further, in accordance with the first program sequence, writing isexecuted in the memory cells in the order of (5), (6), (7), and (8).Subsequently, in accordance with the second program sequence, writing isagain executed in the memory cells in the order of (5), (6), (7), and(8).

Furthermore, as a different writing order, in accordance with the firstprogram sequence, writing is again executed in the memory cells in theorder of (1), (2), (3), and (4) as shown in FIG. 27. Then, in accordancewith the first program sequence, wiring is executed in the memory cellsin the order of (5), (6), (7), and (8). Thereafter, in accordance withthe second program sequence, writing is again executed in the memorycells in the order of (1), (2), (3), and (4). Then, in accordance withthe first program sequence, writing is executed in the memory cells inthe order of (9), (10), (11), and (12). Subsequently, in accordance withthe second program sequence, writing is again executed in the memorycells in the order of (5), (6), (7), and (8).

Moreover, to alleviate read disturbance, when the selection transistorsSGS must be divided like the selection transistors SGD0 to SGD3 butprocessing is difficult, the selection transistors SGS are formed ofdepression type (D-type) (indicated by (D)) and enhancement type(E-type) (indicated by (E)) transistors.

For example, in case of selecting the NAND string NS0, an L-levelvoltage that turns on the D-type of the selection transistor SGS0 andturns off the E-type of the same is applied, and an H-level voltage thatturns on both the D-type and the E-type is applied to the selectiontransistors SGS1 to SGS3, thereby selecting the NAND string NS0 alone.

In case of selecting the NAND string NS1, the L-level voltage that turnson the D-type of the selection transistor SGS1 and turns off the E-typeof the same is applied, and the H-level voltage that turns on both theD-type and the E-type is applied to the selection transistors SGS0,SGS2, and SGS3, thereby selecting the NAND string NS1 alone.

Likewise, in case of selecting the NAND string NS2 alone, the L-levelvoltage can be applied to the selection transistor SGS2, and the H-levelvoltage can be applied to the selection transistors SGS0, SGS1, andSGS3, thereby selecting the NAND string NS2 alone. In case of selectingthe NAND string NS3, the L-level voltage can be applied to the selectiontransistor SGS3, and the H-level voltage can be applied to the selectiontransistors SGS0 to SGS2, thereby selecting the NAND string NS3 alone.

A modification of the third embodiment will now be described.

FIG. 29 is a flowchart showing program sequences according to themodification of the third embodiment. A first program sequencerepresented by (a) in FIG. 29 is the same as the first program sequenceof the third embodiment represented by (a) in FIG. 26. Of a secondprogram sequence represented by (b) and (c) in FIG. 29, a programsequence represented by (b) in FIG. 29 is the same as the second programsequence represented by (b) in FIG. 26, and a program sequencerepresented by (c) in FIG. 29 is a sequence added in this modification.

It is to be noted that, in this example, the second write operation inthe modification of the first embodiment or the second embodiment may becarried out in place of the second program sequence represented by (b)in FIG. 29.

After performing writing to provide data “c” represented by (b) in FIG.29 (a step S19), reading is carried out with respect to memory cellshaving data “a”, “b”, and “c” with the use of read voltage levels A_R,B_R, and C_R, respectively (steps S22, S23, and S24).

Subsequently, whether memory cells corresponding to data stored in theprogram buffer exceed a verify voltage level is determined. That is,whether the number of data latch circuits LDL set to “1” is a specifiednumber or more is determined (a step S25). If the number of the datalatch circuits LDL set to “1” is the specified number or more, thesecond program sequence is terminated.

On the other hand, if the number of the data latch circuits LDL set to“1” is less than the specified number, a write voltage Vpgm is slightlyincreased (step-up), the processing returns to a step S21, and writingof the step S21 and subsequent processing are executed.

Consequently, the write operation and a write verify operation arealternately repeated until the program operation is normally terminated.

In the modification, after end of the second program sequence in thethird embodiment, reading is again executed with the use of the readvoltage levels A_R, B_R, and C_R, and the write operation and the writeverify operation are again repeated if a threshold value of each memorycell has not reached a verify voltage level.

According to the third embodiment and its modification, for example,when the first program sequence is executed with respect to memory cellsin a page or a block and then the second program sequence is executedwith respect to the memory cells in the same page or block, a datacontroller that supplies data to each data latch circuit does not haveto store data of 2 bits in advance, and storing data of 1 bit cansuffice, thus reducing a burden on a control section including the datacontroller.

As described above, according to this embodiment, during the first writeoperation, the data latch circuit that is not used for writing after endof writing to provide a lower threshold value is used as a cache forsubsequent writing. Furthermore, in the additional second writeoperation, before writing, write data is restored by using data readbetween threshold levels of respective data and data of 1 bit generatedfrom the write data of 2 bits, and the second write operation isexecuted based on this restored data. Consequently, before end of thesecond write operation, the data latch circuit can be used as a cachefor subsequent writing.

Fourth Embodiment

FIG. 30 shows a write operation and a write verify operation in aprogram sequence of a semiconductor memory device according to a fourthembodiment.

In the first to third embodiment, the second program sequence operationis performed after the first program sequence. In the program sequencewhere one write operation and the write verify operation are repeated,the write verify is temporarily passed on, and a subsequent writevoltage is supplied to a corresponding memory cell due to programInhibit. If the memory cell turns to Fail in subsequent write verify anda subsequent write voltage is supplied, rewriting can be performed withrespect to the memory cell as writing.

However, since a voltage value of a write voltage Vpgm is increased stepby step in the repetition of the write operation and the write verifyoperation, the write voltage Vpgm for writing is likely to be a highvoltage at this moment, over-program may possibly occurs. Thus, at thetime of the rewriting, an intermediate voltage can be applied to a bitline, and a potential between a gate of a memory cell and a channel canbe alleviated to enable writing. When the second write verify is passedon in this manner, subsequent writing can be non-writing. Configuringsuch a setting enables solving a problem of detrapping by againperforming writing even if the write verify is temporarily passed on andthen a threshold value Vth of a memory cell is lowered due todetrapping.

[Others]

The weak erase operation shown in FIGS. 16, 22, and 24 will now bedescribed. The weak erase operation is executed after the first writeoperation and before the second write operation.

An operation waveform of the weak erase operation will be described withreference to FIG. 31.

Wf1 represents a waveform of a selected word line. The row decodertransfers a voltage VSGD (which is the same as VSGD applied to the writeoperation in this example, but VSGD_RV optimized for the weak eraseoperation may be used) to the gate of the selection transistor ST1during a period from a time ta to a time tg. Additionally, the rowdecoder also transfers a voltage VSS to the gate of the selectiontransistor ST2. Here, since a threshold voltage of each selectiontransistor is approximately 1 to 2 V, if VSGD=2.5 V is set, theselection transistor ST1 enters a conductive state depending on avoltage level in its source terminal (a terminal on a side connected toa memory cell), and the selection transistor ST2 is turned off.

Further, the bit line control circuit 2 applies a voltage VDDSA (e.g.,2.5 V) to a bit line during a period from the time ta to a time tf. In acase where the bit line is being charged with the voltage VDDSA from thetime ta, when the source terminal of the selection transistor ST1 has avoltage raised to “VSGD-Vt_SGD” (Vt_SGD is a threshold voltage of theselection transistor ST1), the selection transistor ST1 enters a cutoffstate.

Then, the word line control circuit 6 raises voltages of a selected wordline and a non-selected word line to VREAD_RV during a period from atime tb to a time td. At this time, since the selection transistor ST1is in the cutoff state, the channel is floating. Thus, the channel ofthe memory cell is boosted by coupling with the voltage VREAD_RV of theword lines WL, and increased to a potential Vch1 (≈VREAD_RV).

Furthermore, after the time td, the word line control circuit 6 keepsapplying the voltage VREAD_RV to the non-selected word line WL, andlowers a potential of the selected word line WL from the voltageVREAD_RV to a voltage VRV (e.g., VSS=0V). Consequently, a potential in acontrol gate of a selected memory cell becomes, e.g., 0 V, and a channelregion of the selected memory cell has a potential Vch1 raised by anon-selected memory cell, and a large potential difference arisesbetween them. As a result, reverse stress can be applied to the selectedmemory cell.

Operation waveforms of another weak erase operation will now bedescribed with reference to FIG. 32.

A weak erase operation shown in FIG. 32 is an example that the word linecontrol circuit 6 applies a voltage (a negative voltage) lower than thevoltage VSS or a voltage lower than a source line voltage as the voltageVRV to a selected word line in the weak erase operation shown in FIG.31. Other basic operation waveforms are the same as those of the weakerase operation shown in FIG. 31.

In the example shown in FIG. 32, since the voltage VRV of a selectedword line is set to a negative voltage at the time of applying reversestress, the reverse stress can be applied by using a channel potentialVch lower than the channel potential Vch1 that must be applied when thevoltage VRV is the voltage VSS. Thus, the voltage VREAD_RV applied to anon-selected word line can be set to be lower than that in case ofsetting the voltage VRV to the voltage VSS. Further, the voltageVREAD_RV can be adjusted to the voltage VREAD at the time of the writeverify.

As described above, according to the embodiment, it is possible toprovide the semiconductor memory device that can suppress an influenceof a fluctuation in threshold value immediately after writing in thememory cells and can realize high-speed writing.

Furthermore, although the example where this embodiment is applied tothe memory cells each of which can store data of 2 bits has beendescribed, the embodiment can be likewise applied to memory cells eachof which can store data of n bits (n is a natural number of 2 or more).

Moreover, this embodiment is not restricted to the NAND flash memory,and it can be applied to other general memory devices. Additionally,although the respective embodiments may be solely carried out, butcombinable embodiments may be combined and carried out.

It is to be noted that each embodiment according to the presentinvention has the following characteristics.

-   (1) In the read operation,

a voltage applied to a word line selected for a read operation of alevel (data “a”) falls within the range of, e.g., 0 V to 0.55 V, or maybe set to any one of the ranges 0.1 V to 0.24 V, 0.21 V to 0.31 V, 0.31V to 0.4 V, 0.4 V to 0.5 V, and 0.5 V to 0.55 V without being restrictedto the former range;

a voltage applied to a word line selected for a read operation of a Blevel (data “b”) falls within the range of, e.g., 1.5V to 2.3 V, or maybe set to any one of the ranges 1.65 V to 1.8 V, 1.8 V to 1.95 V, 1.95 Vto 2.1 V, and 2.1 V to 2.3 V without being restricted to the formerrange; and

a voltage applied to a word line selected for a read operation of a Clevel (data “c”) falls within the range of, e.g., 3.0 V to 4.0 V, or maybe set to any one of the ranges 3.0 V to 3.2 V, 3.2 V to 3.4 V, 3.4 V to3.5 V, 3.5 V to 3.6V, and 3.6 V to 4.0 V without being restricted to theformer range.

A read operation time (tR) may be set to fall within the range of, e.g.,25 μs to 38 μs, 38 μs to 70 μs, or 70 μs 80 μs.

-   (2) The write operation includes the program operation and the    verify operation. In the write operation,

a voltage first applied to a word line selected in the program operationfalls within the range of, e.g., 13.7 V to 14.3 V, or may be set to fallwithin one of the ranges of, e.g., 13.7 V to 14.0 V and 14.0 V to 14.6 Vwithout being restricted to the former range, a voltage first applied toa selected word line at the time of writing data in odd-numbered wordlines and a voltage first applied to a selected word line at the time ofwriting data in even-numbered word lines may be changed.

When the program operation is based on an incremental step pulse program(ISPP) system, a step-up voltage is, e.g., approximately 0.5 V.

A voltage applied to a non-selected word line may be set to fall withinthe range of, e.g., 6.0 V to 7.3 V. This voltage may be also set to therange of, e.g., 7.3 V to 8.4 V or may be set to 6.0 V or less withoutbeing restricted to the former case.

A path voltage to be applied may be changed depending on whether anon-selected word line is an odd-numbered word line or an even-numberedword line.

A write operation time (tProg) may be set fall within the range of,e.g., 1700 μs to 1800 μs, 1800 μs to 1900 μs, or 1900 μs to 2000 μs.

-   (3) In the erase operation,

a voltage first applied to a well that is formed in a semiconductorsubstrate upper portion and has the memory cells arranged thereabovefalls within the range of, e.g., 12 V to 13.6 V. This voltage may be setto the range of, e.g., 1.6 V to 14.8 V, 14.8 V to 19.0 V, 19.0 V to 19.8V, or 19.8 V to 21 V without being restricted to the former case.

An erase operation time (tErase) may be set to a period of, e.g., 3000μs to 4000 μs, 4000 μs to 5000 μs, or 4000 μs to 9000 μs.

-   (4) A configuration of each memory cell has

a charge storage layer arranged on the semiconductor substrate (asilicon substrate) through a tunnel insulating film having a filmthickness of 4 to 10 nm. This charge storage layer may be formed into alamination structure of an insulating film of, e.g., SiN or SiON havinga film thickness of 2 to 3 nm and polysilicon. Further, a metal such asRu may be added to the polysilicon. The charge storage layer has aninsulating film thereon. This insulating film has a silicon oxide filmhaving a film thickness of 4 to 10 nm that is sandwiched between a lowerHigh-k film having a film thickness of 3 to 10 nm and an upper High-kfilm having a film thickness of 3 to 10 nm. The High-k film is made of,e.g., HfO. Furthermore, the film thickness of the silicon oxide film maybe set to be larger than the film thickness of the High-k film. Acontrol electrode having a film thickness of 30 nm to 70 nm is formed onthe insulating film through a work function adjusting material having afilm thickness of 3 to 10 nm. Here, the work function adjusting materialis, e.g., a metal oxide film of TaO or the like or a metal nitride filmof TaN or the like. W and the like can be used for the controlelectrode.

Moreover, an air gap may be formed between the memory cells.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1-15. (canceled)
 16. A semiconductor memory device comprising: a firstmemory cell configured to store first data of n bits by one of first ton-th states (n is a natural number equal to 2 or more); a data storagecircuit electrically connected to the first memory cell and configuredto hold a write data to be written, wherein a state of the first memorycell is written from the first state to one of the first to n-th statesdepending on the data after the first data is inputted to the datastorage circuit in a first write operation, second data of h bits (h<k)generated by the first data is inputted to the data storage circuit in asecond write operation, and data is read from the first memory cellwritten by the first write operation, the first data is restored by theread data and the second data and a second write operation is executedto the first memory cell once again by the restored first data.